In order to achieve a high image quality of an image produced by image capturing of an object space, it is important to reduce noise in the image. Typically, intensity of information of the object space included in the image is lower at a higher frequency, but intensity of the noise in the image is substantially constant irrespective of the frequency. Thus, a region with a higher frequency has a higher fraction of the noise included in an original signal of the image. For this reason, a high frequency component of the image is weakened to reduce the noise in a widely performed noise reduction.
As such a noise reduction disclosed is a method of reducing noise in an image while maintaining edge information of the object space by using a local characteristic of a signal of the image. Patent Document 1 discloses a method of categorizing a local region of an image into an edge part, a flat part and a gradation part and changing a noise reduction parameter depending on these categories. This method reduces the noise in the image while maintaining edge sharpness, by applying a weaker noise reduction in the edge part and a stronger noise reduction in the flat part where the noise is easily noticeable.